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If you have one takeaway from last week's changes in the Dow Jones industrial average, it's this: The line between active and passive investing is extremely thin.

The Dow Jones industrial average swapped out three components for new ones on Sept. 10 — a 10% change in the index's 30 holdings in just one day. And the Standard and Poor's 500-stock index has switched out 241 companies from the index — nearly half — since 1999.

And, according to Dan Wiener, editor of The Independent Adviser for Vanguard Investors, the S&P 400 midcap index has seen more than 400 stocks move out of the index from 2000 through 2009.

"The indexes aren't all the stocks in the world," Wiener says. "There are real people picking stocks to go into the index. It's a choice to be in the S&P 500 and the Dow."

Many people use index funds, which ditch the manager and simply follow an index. These funds have extremely low costs, and most are tax-efficient, meaning that they pay out little in the way of capital gains distributions. And, while they do less trading than funds with managers, they're not as passive as investors might think.

The Standard and Poor's 500-stock index, one of the most widely tracked stock indexes in the world, doesn't make as sweeping one-day changes, as the Dow did last week. But it does make a number of smaller ones. For example, Standard and Poor's announced last week that two components — chipmaker AMD and engineering firm SAIC — would be dropped from the S&P 500-stock index, to be replaced by Vertex Pharmaceuticals and Ametek, an electronics firm. The changes take place Sept. 20.

"Every one of the stocks in the S&P 500 or the S&P 400 is chosen by a committee," Wiener writes in his newsletter. "These are not passive by any means but — as you can tell — heavily active."

Not all changes in the S&P 500 or the Dow occur because the committee decides to change them. In some cases, mergers or acquisitions will take a stock out an index.

The S&P indexes change by weighting, too. Stocks with the highest capitalization — market price multiplied by shares outstanding — get the biggest weight in the index. The top holding in the S&P 500 today is Apple, followed by ExxonMobil and Google. In 1999, the three top holdings were Microsoft, General Electric and Cisco.

The Dow Jones industrial average shed three stocks, or 10% of its holdings, last week, the biggest reshuffling of the index since 2004. The Dow ditched Bank of America, Hewlett-Packard and Alcoa, replacing them with Goldman Sachs, Visa and Nike.